1. Field of the Invention
The present invention relates to gas sensors.
2. Description of the Related Art
To date, a gas sensor that detects the concentration of a specified gas such as NOx included in an object gas such as an exhaust gas from an automobile has been known. For example, Patent Literature 1 discloses a gas sensor including a long plate-shaped sensor element formed of a plurality of gas-tight oxygen ion conductive solid electrolyte layers stacked one on top of one another. In this sensor element, a measurement electrode and a reference electrode are formed on separate solid electrolyte layers. An object gas is introduced on the measurement electrode side and a reference gas is introduced on the reference electrode side, and the concentration of a specified gas in the object gas is detected in accordance with an electromotive force generated between the reference electrode and the measurement electrode.
FIG. 6 is a schematic sectional view generally illustrating the structure of an example of such a related-art gas sensor 300. As illustrated in FIG. 6, the gas sensor 300 includes a sensor element 307. The sensor element 307 has a structure in which oxygen ion conductive dense solid electrolyte layers 301 to 306 are stacked one on top of one another. In the sensor element 307, an object gas flow path 310 is formed between a lower surface of the solid electrolyte layer 306 and an upper surface of the solid electrolyte layer 304. The object gas is introduced through the object gas flow path 310. A reference gas introduction space 343 is formed between an upper surface of the solid electrolyte layer 303 and a lower surface of the solid electrolyte layer 305. The reference gas is introduced through the reference gas introduction space 343. The concentration of a specified gas in the object gas is detected with reference to the reference gas. A measurement electrode 344, which is a porous cermet electrode, is formed directly on the upper surface of the solid electrolyte layer 304 that faces the object gas flow path 310. A reference electrode 342 is formed directly on the upper surface of the solid electrolyte layer 303 that faces the reference gas introduction space 343. The reference electrode 342 is covered with a reference gas introduction layer 348 formed of a porous body. The reference gas is introduced from the reference gas introduction space 343 through the reference gas introduction layer 348. The concentration of the specified gas in the object gas introduced into the object gas flow path 310 is detected in accordance with an electromotive force Va generated between the measurement electrode 344 and the reference electrode 342. A heater 372 is formed between the solid electrolyte layer 302 and the solid electrolyte layer 303. The heater 372 is insulated from the solid electrolyte layers 302 and 303 using a heater insulation layer 374. The heater 372 heats and increases the temperatures of the solid electrolyte layers 303 and 304 to cause the solid electrolyte layers 303 and 304 to be activated, thereby increasing the electrical conductivity of oxygen ions between the measurement electrode 344 and the reference electrode 342. A pressure releasing hole 375 is formed in the solid electrolyte layer 303 in order to suppress an increase in an internal pressure occurring as the temperature in the heater insulation layer 374 increases.    Patent Literature 1: JP 2006-284223 A